1. The Field of the Invention
This invention pertains to building construction, and, more particularly, to novel methods and apparatus for anchoring building walls to foundations and lower floors thereof. The invention provides an automatic adjusting mechanism to remove slack in a hold down system caused by wood shrinkage over time or wood crushing caused by earthquakes.
2. The Background Art
Wood products change dimensions as moisture content changes. Floor systems using solid sawn joists typically shrink approximately five percent in dimensions across the grain. Under certain conditions they have been known to shrink six and one-half percent within a year. This shrinkage is typically part of the overall process and condition called “settling.” Settling actually includes both settling of foundations, as well as settling of walls due to shrinkage.
Testing and load rating has been completed for shear walls mounted to solid underlying surfaces. The solid surfaces are typically comprised of steel, concrete, or both. In tests wherein a wall is constructed, and immediately tested thereafter, test results are substantially better than those for walls that have existed over time. In a typical practice, a sill plate anchor or lower anchor is a threaded rod or an anchored strap capturing the base plate or sill plate of a wall (the bottom, horizontal member above which the studs extend vertically). Over time, ranging from several months to several years, wood loses moisture, shrinks, and the building settles. Threaded rod type anchors become loose. Strap type anchors buckle if positively engaged and become loaded in compression, or the like.
Current tiedown systems (including rods, straps, and the like) do not provide a solution for this problem. After a building “settles” the wall can lift before it will re-engage the hold down structure before the tiedown is even loaded to begin resisting movement of the wall. Substantial building damage can result before the anchoring hardware is loaded (in tension). Hardware that does not immediately engage the base of an anchored wall can result in a 50 percent to 70 percent loss in lateral, load-bearing capacity.
The problem arises, typically, in wind storms of great power, or in earthquake conditions. A building under such circumstances may be violently loaded or shaken back and forth in a lateral direction with respect to the extent of the wall. If a shearwall is tightly restrained by its base to a foundation, loads may be smoothly transferred from a horizontal to a vertical direction. Loads are resolved in the foundation, where they appear as tension and compression forces.
Buildings are often composed of long walls, (walls with a length greater than the height) and short walls (walls that have a length shorter than the height). The uplift load on a particular wall is inversely proportional to the length of the wall. Tall narrow shear walls (as commonly found in nearly all homes) act as lever arms and tend to magnify the input load. In certain instances and depending upon wall structural configuration, the actual load on the anchoring system may be magnified to several times the original load. Gaps caused by wood shrinkage may further introduce an undesirable shock load to the anchoring system as the gaps are closed and the anchor system is finally loaded.
However, the as-built building is generally not the building that will be sustaining loads induced by earthquake shaking or by wind. Wood components of the building structure, including floors, sill plates, top plates, and studs, will shrink. Shrinkage varies greatly but it ranges typically from about one-quarter inch under the best of conditions, to well over one inch.
Moreover, under load, wood crushes or collapses in compression under the loading of a wall. Neither shrinkage nor crushing are well-accommodated or otherwise resolved in currently available systems. These problems lead to a significant reduction in the lateral, load-bearing capacity of shearwalls. Typically, based on testing, load-bearing capacity reductions range from about 30 percent to about 70 percent, depending on whether the rating used corresponds to building codes for property preservation, or life safety.
A better hold down or tiedown system including an improved take-up is needed to accommodate shrinkage of building materials. An improved tiedown system with such an improved take-up mechanism will improve the strength of shear walls subject to shrinkage of constituent materials.